Wireless communications systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or other multiple access techniques. For example, such systems can conform to standards such as Third-Generation Partnership Project 2 (3GPP2, or “CDMA2000”), Third-Generation Partnership (3GPP, or “Wideband Code Division Multiple Access (W-CDMA)”), or Long Term Evolution (“LTE”).
Voice over LTE (VoLTE) and Voice over HSPA (VoHSPA) are two technologies which enables voice services in LTE and 3G High Speed Packet Access (HSPA) networks based on IP Multimedia Subsystem (IMS). To provide voice call continuity for end users moving between an IMS-voice-supported area and a non-IMS-voice-supported area, 3GPP introduces two inter-radio-access-technology (inter-RAT) handover procedures, the SRVCC handover procedure and the rSRVCC handover procedure, to ensure that voice calls can be handed over between IMS and legacy systems in a seamless manner.
The SRVCC handover procedure allows an IMS call (which can be a VoLTE call or a VoHSPA call) to be transferred from IMS to the legacy system and become a Circuit Switched (CS) call in the 2G or 3G network. While the SRVCC handover procedure is a PS-to-CS handover procedure, the rSRVCC handover procedure on the contrary is a CS-to-PS handover procedure which allows a CS call in the 2G or 3G network to be transferred from a legacy system to IMS and become a VoLTE call or a VoHSPA call.
When a user equipment (UE) with an ongoing call is moving between an IMS-voice-supported area and a non-IMS-voice-supported area, the network can trigger SRVCC or rSRVCC handover procedures to ensure that the call can be continued between IMS and the legacy system. Taking VoLTE as an example, when a UE with an active VoLTE call is moving out of the coverage of the LTE network, the network can trigger an LTE-to-2G SRVCC handover procedure to hand over the UE from the LTE network to the 2G network and at the same time make the VoLTE call become a CS call in the 2G network.
The SRVCC and rSRVCC handover procedures are actually an access domain transfer procedure for voice calls. When a voice call on the IMS domain is being transferred to the CS domain through an SRVCC handover procedure or a voice call on the CS domain is being transferred to the IMS domain through an rSRVCC handover procedure, the access domain of the voice call is temporarily uncertain to the UE because the handover procedure has not been completed. It depends on the result of the handover procedure (i.e., success or failure) that decides whether the access domain of the voice call is changed or not. As the access domain of a call during an SRVCC handover procedure or during an rSRVCC handover procedure is uncertain, how to handle a voice call when the UE receives a request for disconnecting the voice call, or receives a request for placing the voice call on hold from an end user during an SRVCC handover procedure or an rSRVCC handover procedure is an important issue to be researched and discussed.